We report on an antigen (Mr 39,500) that is expressed in vitro by spirochetes of Borrelia garinii strain IP90 and apparently only in vivo by spirochetes of the B. burgdorferi sensu stricto strain JD1. Antibody against this 39.5 kDa antigen kills IP90 spirochetes in vitro by antibody-dependent, complement-mediated killing (ADCK) regardless of whether the antibody was obtained by 1) affinity purification using as immunoabsorbant native P39.5 antigen of IP90 spirochetes (as separated on a Western blot) and as source of antibody the antiserum generated during an infection of rhesus monkeys with JD1 spirochetes or, 2) by immunizing mice with recombinant P39.5 from IP90 (rP39.5). The mouse anti-rP39.5 antibody does not kill JD1 spirochetes in vitro by ADCK. A segment of 1190 bp of the coding region of the p39.5 gene of IP90 was sequenced. It encompasses a single open reading frame that encodes a putative 37.7 kDa protein. Both ends of the gene have not been sequenced as yet. The sequenced fragment is composed of mostly hydrophilic domains that contain several internally repeated regions; the corresponding deduced amino acid sequence is up to 22% identical to members of the Vmp family of outer surface lipoproteins of Borrelia hermsii. The native P39.5 was fully extracted into the detergent phase in a Triton-X114 extraction experiment. Therefore, P39.5 is probably a lipoprotein. The internal repeats of P39 indicate that this protein, like OspA, may undergo homologous recombinations in Borrelia which may lead to mutants expressing shortened, partially-deleted versions of the gene. While this could be a cause of concern for a vaccine antigen, we reported last year that OspA deletion mutants that are resistant to antibody alone (escape mutants) can be killed with antibody plus complement. Hence, deletion mutants of vaccine antigens that, like P39.5 and unlike OspA, are expressed in vivo, will be killed upon infection of the vertebrate host.